In amplifiers, in particular power amplifiers, feedback paths are provided to flatten the gain curve and to assure stability of operation with regard to oscillations. Such feed-back paths comprise feedback lines in the form of conductors in connection with feed-back circuit elements, i.e. resistors, inductors, and capacitors, wherein a combination of the latter forms an application-specific feedback element. The use of at least one capacitor in such a feedback path is necessary in order to block the flow of direct current between the input and output of the transistor package, as such a flow would lead to short-circuiting of the transistor package.
Suitable feedback paths for a given application can achieve improvements of bandwidth and gain ripple versus frequency as well as stabilize individual stages of multi-stage amplifiers, e.g. preamplifiers, driver amplifiers and also, especially when lower output levels are required, final amplifiers of modular amplifiers.
Amplifiers of the above-mentioned type use bulky transistors, i.e. being relatively big with regard to the used wavelengths, hereinafter referred to as packaged transistors or transistor packages, which are used as surface mounted devices (SMDs) or drop-in devices in hybrid circuit layouts and which require an extensive amount of circuit board space. There are known transistor packages with fully integrated feedback paths, i.e. the individual feedback loops are integrated in the transistor packages, which can be used to build up the amplifier. Normally, subsequent modification of the feedback elements is not possible when implementing this type of integrated feedback solution.
However, for the majority of applications, only packaged transistors without internal feedback paths are available. If a feedback path is needed for a particular application, this problem can only be avoided by either devising an external realisation for the feedback path or by developing an expensive customer specific transistor.
Due to the large sizes of the transistor packages, known amplifiers use external feedback paths comprising printed feedback lines on the circuit board which are relatively long and thus not well suited for high frequency applications, e.g. RF (radio frequency)-applications in the GHz-range. Long lines in feedback paths have a negative effect on the performance of the amplifier. Therefore, such lines should be avoided. A transmission line is referred to as “long” if it is longer than about 1/10 of the RF wavelength being used. Long lines cause impedance transformation and may therefore lead to an undesirable misbehaviour of the feedback path, e.g. if an inductive element is used as a feedback element it may be transformed into a capacitive element because of the long line.